This invention relates to the extrusion of plastic pipe. In one of its aspects this invention relates to the extrusion of thick-walled, large diameter plastic pipe. In another of its aspects this invention relates to providing smooth exterior surfaces on extruded pipe.
In the extrusion of plastic pipe one of the problems most often encountered is the production of unsightly weld lines on the outside surface of the extrusion. As the diameter of the pipe increases, the thickness of the pipe wall is also usually increased to provide the necessary strength and rigidity in the final product. As pipe increases in diameter the mixing and blending of molten polymer merging from individual channels into a choke zone can be sufficiently poor that weld lines on the external surface become noticeable.
It is, therefore, an object of this invention to provide extruded pipe, particularly of large diameter, having superior external surface appearance. It is another object of this invention to provide apparatus and method for extruding plastic pipe with superior blending and welding of the polymer flow through a choke ring.
Other aspects, objects, and the various advantages of this invention will become apparent upon studying this specification, the drawing, and the appended claims.